This invention relates to an earth-based receiving station for receiving radio signals from a satellite relay station.
In an SCPC (Single Channel Per Carrier) system used as a satellite communication system, voice channels are allocated with 45 kHz separation. Since this separation is very narrow for high carrier frequencies in GHz band, and since utmost frequency accuracy is required for demodulating PSK modulated signals used in satellite communication, frequency tuning should be very accurate in receivers for receiving these channels.
In an SPADE (Single channel per carrier PCM multiple Access Demand assignment Equipment) system, the control station assigns a communication channel in accordance to access demand from an earth-based transmitting station. For this demand/assign communication between the control station and earth-based communication stations, a CSC (common signalling channel) is used.
In order to facilitate accurate tuning, the CSC is continually transmitted from the control station.
Earth-based transmitting stations transmit access demand signals to the control station by CSC in the up-linked channels.
Earth-based receiving stations in the system are listening CSC transmitted from the control station, and when the control station assigns a receiving channel by CSC to a receiving station, the receiving station tunes a channel receiver frequency to the assigned channel.
All the communication channels are transmitted in a burst type waves.
FIG. 2 illustrate a block diagram of such receiving station of a prior art.
Radio frequency signals received by an antenna 1 are converted to first intermediate frequency signals by a first frequency converter 2. The first intermediate frequency signals from the converter 2 are supplied through a distributor 3 to a control receiver 5 and a channel receiver 6.
Same numerals in the receivers 5 and 6 indicate same parts, and as for receiver discrimination, a numeral indicating the receiver is connected by a hyphen.
The control receiver 5 receives the CSC, phase-locks a VCO 44 to the CSC carrier frequency, and delivers output of the VCO 44 as a first local oscillator frequency to the converter 2. A phase-lock loop for the VCO 44 is a closed circuit comprising VCO 44, first frequency converter 2, second frequency converter 40-5, second intermediate frequency amplifier 41-5, demodulator 42-5, low-pass-filter 43, VCO 44. Frequency synthesizer 48-5 supplies a second local oscillator frequency to the second frequency converter 40-5.
When the CSC carrier frequency is F.sub.pr, the first local oscillator(VCO) frequency is F.sub.1, the second local oscillator frequency is F.sub.2p, center frequency of the second intermediate frequency amplifier is F.sub.2, and a demodulator 42-5 detects phase error of output of the second intermediate frequency amplifier 41-5, the VCO is phaselocked to the CSC carrier frequency in a relation F.sub.pr -F.sub.1 -F.sub.2p =F.sub.2 . . . (1). Since pull-in range of the phase-lock loop is very narrow, a crystal controlled oscillator is used as the VCO 44.
An amplitude level at an output of the second intermediate frequency amplifier 41-5 is detected by a level detector 45. The detected level is supplied to the converter 2 for feed-back controlling gain of the converter 2.
Thus, all the channel frequencies in a radio frequency band are accurately converted to a first intermediate frequency band. The first intermediate frequency signals are supplied to the channel receiver 6, and a receiving frequency of a channel receiver is determined by the second local oscillator frequency which is supplied from a synthesizer 48-6.
The channel receiver 6 is listening CSC, and when the channel receiver 6 receives a command on CSC for assigning a channel frequency, this command is demodulated by a demodulator 42-6, is decoded by a decoder in a data processor 46, and controls a logic 47-6 in accordance with the assigned channel. The logic 47-6 changes output frequency of the synthesizer 48-6 to generate a second local oscillator frequency to tune for receiving the assigned channel frequency.
When an earth-based receiving station receives plural communication channels simultaneously, plural channel receivers 6 are provided as shown in FIG. 3.
There is a vulnerability in the earth-based receiving station of the prior art. When the control receiver 5 is in a trouble, and the first local oscillator frequency supplied from the VCO 44 is unreliable, all the channel receivers 6 suffer the trouble. And when a standing spare is provided for the control receiver to avoid the vulnerability, the standing spare can not be used as a channel receiver even when all other channel receivers are busy.